1. Field of Invention
This invention relates generally to methods and devices for measuring characteristics of an optical signal. More particularly, it relates to devices and methods for determining wavelength, power and optical noise of a plurality of optical signals in an optical fiber.
2. General Background and Related Art
In recent years, optical communications systems have become more important in many types of networks. Local area networks, wide area networks, telecommunications systems, CATV (variously used to indicate; community antenna television systems and cable television systems) and other types of systems make use of optical communications technologies.
A somewhat recent development in optical communications systems is the use of wavelength division multiplexed (WDM) systems. Such systems make use of multiple wavelength channels of light carried in a single mode optical fiber to transmit information. Such systems are generally capable of handling a great deal more information than systems employing only a single wavelength channel of light on each fiber.
In general, WDM systems transmit on single mode optical fibers having a low-dispersion window. Currently, more and more wavelength channels are being transmitted within a given bandwidth in the low-dispersion window. A system using such closely packed channels is known as dense WDM (DWDM). In WDM systems, as well as in other types of optical communication systems, it may become necessary to monitor each transmitted channel in order to determine the wavelength and power of each channel and to determine the optical noise intensity near each source wavelength. Such monitoring becomes increasingly important as one increases the number of wavelength channels transmitted through the fiber. The information produced by the monitor may be used, for example, for feedback control of the light source or sources, to control signal quality during environmental changes and reconfigurations of the system, to monitor instability in various components of the system, or to ensure that a selected channel is the correct one.
A limited form of wavelength monitoring is disclosed by Villeneuve et al. in U.S. Pat. No. 5,825,792, herein incorporated by reference. Villeneuve discloses the use of a Fabry-Perot filter disposed within an optical path of the signal to be measured. The filter is positioned at an angle so that different wavelengths of light are transmitted at varying angles with respect to the incoming light. A pair of photodiodes is used to detect light at two pre-selected angles, and the output of the photodiodes is used to provide an electrical signal for feedback control of the light source.
Another approach is disclosed by Mizrahi et al in U.S. Pat. No. 6,111,681, herein incorporated by reference. Mizrahi teaches a method and apparatus for providing a stabilized optical selector. The device of Mizrahi correlates a wavelength of a wavelength selector to a wavelength emitted by an optical transmission source. The optical transmitter includes a wavelength reference which is coupled to a feedback loop. The feedback loop uses thermal control to adjust the output of the transmitter. A tap in the transmission line diverts a portion of the transmitter""s energy to a wavelength reference, such as a Bragg grating. A photodiode detects light transmitted through the Bragg grating and provides a feedback signal to a microprocessor which directs the thermal control of the transmitter. Mizrahi et al. likewise disclose the reverse device, providing a feedback loop for thermal or strain control of the reflection wavelength of the Bragg grating, using the laser source as the reference.
Davis et al. (U.S. Pat. No. 5,818,585, herein incorporated by reference) disclose another system for monitoring of reflected wavelengths from multiple strings of fiber Bragg gratings (FBG) using a scanning optical filter and a reference string of FBG elements. As is the case with the device of Mizrahi et al., the reference does not provide an absolute measurement since the FBG references themselves have a dependence on physical parameters such as temperature and strain which may vary over time.
The details of operation of a Fabry-Perot interferometer, and its use as an optical filter can be gleaned from U.S. Pat. 5,392,117 issued to Belleville et al., herein incorporated by reference.
The present invention provides methods and devices for monitoring optical signals.
An optical channel monitor according to an embodiment of the present invention includes a tunable optical bandpass filter in optical communication with an optical input, a photodetector in optical communication with the optical input, a reference light source in optical communication with the tunable optical bandpass filter, configured and arranged to produce a reference light beam having a bandwidth greater than a passband width of the tunable optical bandpass filter, a reference gas cell containing a reference gas and in optical communication with the reference light source such that at least a portion of the light from the reference light source passes through both of the reference gas cell and the tunable optical bandpass filter and the reference gas has a plurality of known absorption bands.
In one embodiment, the above described optical channel monitor makes use of a single photodetector which selectively or simultaneously receives light from the input optical signal after passing through the filter and light from the reference light source after passing through the filter and the reference gas cell. In another embodiment, one photodetector is used to receive light from the input optical signal after passing through the filter and another is used to receive light from the reference light source after passing through the filter and the reference gas cell.
A method of measuring characteristics of an optical signal in accordance with the present invention includes directing at least a portion of the optical signal to a tunable bandpass optical filter, filtering the optical signal with the tunable bandpass optical filter, providing a reference optical beam having components within a bandwidth wider than the wavelength passband of the tunable bandpass optical filter, passing the reference optical beam through a reference gas cell and the tunable filter to produce a reference optical signal modulated by absorption by the reference gas and detecting the reference optical signal and the filtered optical signal.
A wavelength division multiplexed optical communication system according to the present invention includes a transmitter, an optical transmission line in communication with the transmitter, an optical channel monitor in optical communication with the optical transmission line and including an optical input, a tunable optical bandpass filter in optical communication with the optical input, a photodetector in optical communication with the optical input, a reference light source in optical communication with the tunable optical bandpass filter and configured and arranged to produce a reference light beam having a bandwidth greater that a passband width of the tunable optical bandpass filter and a reference gas call containing a reference gas having a plurality of known absorption wavelength bands and disposed to receive the reference light beam after the reference light beam passes through the filter, and a receiver in optical communication with the transmission line.